1. Field of the Invention
The present invention relates to a liquid crystal display panel used as a display unit of an information apparatus, a method of inspecting the same, and an inspection apparatus used for the same.
2. Description of the Related Art
Active matrix liquid crystal displays having a thin film transistor (TFT) as a switching element at each pixel are attracting attention as the main stream of flat panel displays, and it is needed to reduce the cost of such displays by improving the yield of manufacture, reducing product failures, and simplifying manufacturing processes. An active matrix color liquid crystal display includes a TFT substrate having TFTs formed thereon, a CF substrate having color filters (CFs) formed thereon, and a liquid crystal sealed between the substrates.
At a substrate combining step of a liquid crystal display manufacturing process utilizing the dip type vacuum filling method, a sealing material is applied to a peripheral portion of either TFT substrate or CF substrate, and the substrates are pressurized and combined using a substrate combining apparatus such as a pressurizing/heating apparatus or vacuum heating apparatus to fabricate each combination of substrates having a predetermined cell gap. Thereafter, at a liquid crystal filling step, the gap between the combined substrates is filled with a liquid crystal, and a liquid crystal filling port is sealed.
The recent trend toward substrates in greater sizes has resulted in a problem in that the dip type vacuum filling method has a difficulty in forming a cell gap with high accuracy and takes a long time to fill the gap with a liquid crystal. A method for solving the problem is the one drop filling method (dispensing and combining). According to the one drop filling method, a sealing material is continuously applied to the entire periphery of either of substrates, and a prescribed amount of liquid crystal is dispensed onto the substrate or the other substrate. Subsequently, after the substrates are combined in vacuum, they are put back in the atmospheric pressure to fill a liquid crystal between them. The sealing material is thereafter cured. The use of the one drop filling method allows substrate combining and liquid crystal filling to be completed substantially at the same time and allows steps for manufacturing a liquid crystal display to be simplified significantly.
The cell gap of a liquid crystal display panel fabricated using the one drop filling method is determined by the amount of the liquid crystal dispensed. When the height of pillar spacers formed on either of the substrates is greater than the cell gap, a bubble (vacuum bubble) is generated in the liquid crystal display panel. Since the height of the pillar spacers can vary depending on various factors, it is difficult to prevent the generation of a bubble completely. A bubble tends to be generated in a picture-frame area located outside a display area in general, and a liquid crystal display panel having a bubble in the picture frame area is regarded as a defective panel because a bubble will be generated also in the display area at low temperatures. Therefore, there is a need for an in-process inspection to check whether a liquid crystal display panel has a bubble in the picture-frame area thereof or not.
At a liquid crystal filling step employing the one drop filling method, uncured sealing material can contact a liquid crystal because substrate combining and liquid crystal filling is performed simultaneously. The liquid crystal can be contaminated when an uncured component of the sealing material is in contact with the liquid crystal for a long time or exposed to a high temperature in such a state. Therefore, in general, a heat-curing resin is not used as the sealing material when the one drop filling method is employed, and a photo-curing resin which is quickly cured by being irradiated with ultraviolet light (UV light) is used instead. UV light is radiated from the side of a CF substrate formed with no metal wiring which blocks light. Since a light-shield mask (or black matrix (BM)) is formed in a picture-frame area of the CF substrate, the sealing material is applied to an outside where the BM is located. However, the BM and the sealing material can overlap each other as a result of misalignment of the substrate at a sealing material application step. Even a partial overlap between the BM and the sealing material can result in contamination of the liquid crystal because the sealing material is not irradiated with UV light and is therefore uncured in such a part. It is therefore necessary to perform an in-process inspection to check whether there is an overlap between the applied sealing material and the BM or not.
As thus described, a liquid crystal display panel fabricated using the one drop filling method must be subjected to in-process inspections on two inspection items with respect to the picture frame thereof, i.e., whether there is any bubble or not and whether there is any overlap between the sealing material and the BM. However, inspections on the two items inevitably rely on visual check by skilled inspectors. Therefore, when 100% inspection is to be carried out, the in-process inspections constitute a heavy burden and result in a problem in that the inspections can make liquid crystal display panel manufacturing steps complicated and can increase the manufacturing cost.
Patent Document 1: Japanese Patent Laid-Open No. JP-A-2003-280031